At present, propylene is mostly isolated from the product mixture formed in the steam cracking of light naphtha. Economic and other reasons make it desirable to have a more flexible raw material basis. An alternative to isolation of propylene from mixtures in which it is present is the dehydrogenation of propane.
As a non-oxidative route, propylene can be obtained by dehydrogenation of propane over noble metal catalysts such as Pt/Al2O3, Pt/Sn/Al2O3 or over noble metal-free catalysts such as Cr/Al2O3. The reaction is strongly endothermic and proceeds at a satisfactory rate only at a high temperature. This promotes secondary reactions, eg. degradation of the propane to form ethylene and methane; at the same time, ethylene is hydrogenated by the hydrogen liberated in the dehydrogenation. The selectivity of the reaction decreases greatly with increasing conversion because of the by-product-dependent competing reactions, which makes the industrial implementability of the process questionable. In addition, secondary reactions lead to carbon deposits on the catalysts used, which would have to be regenerated after relatively short periods of operation.
In a process which has achieved industrial maturity, the dehydrogenation is carried out at low pressure and relatively high temperature and the catalyst is continuously regenerated using atmospheric oxygen (Energy Prog. (1986), 6(3) 171-6 and Chem. Eng. Today, Copying Uncertainty, Aust. Chem. Eng. Conf. 11th (1983), 663-71). The process can be carried out using Pt/Al2O3 catalysts in a moving bed at 600-700xc2x0 C. and a pressure of 2-5 bar.
The process described in WO 9523123 uses Cr/Al2O3 catalysts which are operated cyclically, ie. using a regenerative procedure. In this process, the propane is preheated using the waste heat liberated in the burning-off of the carbon. Pt/Sn/Al2O3 catalysts are known from Shiyou Huagong (1992), 21(8), 511-515. That reference also discloses that these catalysts can be doped with potassium or magnesium. Doping with tin is said to slow the deactivation, despite formation of carbon deposits (Stud. Surf. Sci. Catal. 1994, 88, 519-24).
Oxidic catalysts comprising redox-active elements which are not present in their lowest oxidation state are described in EP-A-403 462.
The dehydrogenation of propane using zeolites of the ZSM-5 type is likewise known. If these zeolites are doped with zinc, this influences the acid-base behavior of the zeolites: cracking reactions are said to be largely suppressed (J. Chin. Inst. Chem. Eng. (1990), 21(3), 167-72).
The processes which have become known have, in particular, the disadvantage that the selectivity decreases greatly with increasing conversion. In addition, the catalysts have to be regenerated frequently, which is extremely disadvantageous for an industrial process.
It is an object of the present invention to remedy the above-mentioned disadvantages and to provide catalysts which make possible a process for preparing, in particular, propylene and other low molecular weight olefins by dehydrogenation of corresponding paraffinic hydrocarbons and which achieve a high selectivity even at a high conversion.
We have found that this object is achieved by the use of catalysts based on ceramic oxides of transition group IV of the Periodic Table of the Elements, preferably having more than 90% in one crystalline modification, which can comprise a dehydrogenation-active element and possibly further elements.
Suitable ceramic oxides are, in particular, zirconium oxide (ZrO2) and titanium oxide (TiO2). The ceramic oxide can be doped with from 0.005 to 5% by weight of a metal of transition groups VI and VIII such as palladium, platinum and/or rhodium. Suitable dehydrogenation-active elements are especially metals of transition group VIII, with the noble metals platinum and palladium being particular suitable; preference is given to platinum.
If a noble metal is used as dehydrogenation-active element, it is possible to make additional use of from 0.005 to 5% by weight of metals which can slow the sintering of the noble metal, for example rhenium, Ir and Sn, in particular Re and Sn.
Possible further elements are those which are known to be able to influence the acidity of the catalyst surface or to stabilize noble metals against sintering. Such further elements are all elements of main groups I and II, ie. Li, Na, K, Rb, Cs on the one hand and Mg, Ca, Sr and Ba on the other hand. Suitable elements of main group III are, in particular, gallium, indium and thallium. Suitable elements of transition group III are, in particular, Y and La and also rare earth elements. Zinc has also been found to be effective.
The use of the ceramic oxides of transition group IV is essential for the purposes of the present invention, while the other constituents are only of importance for the base reaction and play a supporting role. Thus, other dehydrogenation-active metals, for example from transition group VI, in particular chromium or molybdenum, can be present in place of a noble metal.
It is essential for the present invention that the crystalline phase of the zirconium oxide is stable under the conditions of the dehydrogenation. If tetragonal ZrO2 is employed, this can be stabilized by doping with La or Y.
The pore width of the catalysts is preferably from 2 to 60 nm, where 10% of the pores have a width of more than 20 nm and the specific pore volume is from 0.1 to 1 ml/g.
Compared with the known catalysts, the catalysts of the present invention have the advantage of higher selectivity with simultaneously higher conversion in the dehydrogenation of propane to propylene. In addition, a further advantage found is that the catalysts of the present invention can be operated without additional hydrogen which would otherwise have to be used for suppressing the formation of carbon deposits. Further advantages are their high mechanical strength, high operating lives and easy shaping.
To prepare the catalysts of the present invention, it is possible to use amphoteric oxides of zirconium and titanium or their mixtures or suitable precursors which can be converted into the oxides by calcination.
The preparation process can be selected from among known model processes, for example the sol-gel process, precipitation of the salts, dehydration of the corresponding acids, dry mixing, slurrying or spray drying.
The doping with a basic compound can be carried out either during the preparation, for example by coprecipitation or subsequently by impregnation of the ceramic amphoteric oxide with a compound of the relevant alkali metal or alkaline earth metal compound, etc.
The dehydrogenation-active constituent is generally applied by impregnation with a suitable compound of the element concerned. Such a compound is selected so as to be able to be converted into the corresponding metal oxide by calcination. However, instead of impregnation, the dehydrogenation-active component can also be applied by other methods such as spraying. Suitable metal salts are, for example, the nitrates, acetates and chlorides of the corresponding metals; also possible are complex anions of the metals used. Preference is given to using H2PtCl6 or Pt(NO3)2 for platinum and Cr(NO3)3 or (NH4)2CrO4 for chromium. Suitable precursors when using noble metals as dehydrogenation-active components also include the corresponding noble metal sols which can be prepared by one of the known methods, for example by reduction of a metal salt with a reducing agent in the presence of a stabilizer such as PVP. The preparation is described in detail in DE 195 00 366.
The catalyst can be used in a fixed bed or, for example, in the form of a fluidized bed and have an appropriate shape. Suitable shapes are, for example, granules, pellets, monoliths, spheres or extrudates having an appropriate cross-section, eg. wagon wheel, star, ring.
The content of alkali metal, alkaline earth metal or a metal of main group III or transition group III or a rare earth metal or zinc is up to 20% by weight, preferably from 1 to 15% by weight, particularly preferably from 1 to 10% by weight. Alkali metal and alkaline earth metal precursors used are advantageously compounds which can be converted directly into the corresponding oxides by calcination. Suitable examples are hydroxide, carbonate, oxalate, acetate or mixed hydroxycarbonates.
If the ceramic support is additionally or exclusively doped with a metal of main group III or transition group III, starting compounds in this case too should be ones which can be converted into the corresponding oxides by calcination. If lanthanum is used, examples of suitable compounds are lanthanum oxide carbonte, La(OH)3 La3(CO3)2, La(NO3)3 or lanthanum compounds containing organic anions, eg. lanthanum acetate, lanthanum formate or lanthanum oxalate.
The content of a dehydrogenation-active component in the catalysts is up to 10% by weight. It is also possible to use catalysts which contain no dehydrogenation-active element. If the catalyst is doped with a dehydrogenation-active element of transition group VIII as dehydrogenation-active element, the content is from 0 to 10% by weight, preferably from 0.2 to 8% by weight, particularly preferably from 0.5 to 2% by weight. If the catalyst is doped with a noble metal as dehydrogenation-active component, the content is from 0 to 5% by weight, preferably from 0.2 to 2% by weight, particularly preferably from 0.5 to 1.5% by weight.
The catalysts have a BET surface area of up to 500 m2/g or more, preferably 10-300 m2/g, particularly preferably 20-100 m2/g. The pore volume is generally from 0.1 to 1 ml/g, preferably from 0.15 to 0.6 ml/g, particularly preferably from 0.2 to 0.4 ml/g. The mean pore diameter, which can be determined by Hg penetration analysis, is from 0.008 to 0.06 xcexcm, preferably from 0.01 to 0.04 xcexcm.
The dehydrogenation of propane is carried out at 300-800xc2x0 C., preferably 450-700xc2x0 C., and at pressures at from 10 mbar to 100 bar, preferably from 100 mbar to 40 bar, at a WHSV (weight hourly space velocity; in [(g of starting material)xc2x7(g of cat.)xe2x88x921xc2x7hxe2x88x921] of 0.01 to 100, preferably from 0.1 to 20. Besides the hydrocarbon to be dehydrogenated, diluents such as CO2, N2, noble gases or steam may be present. If appropriate, ie. under severe reaction conditions, hydrogen can be added to the hydrocarbon stream; the ratio of hydrogen to hydrocarbon stream can be from 0.1 to 100, preferably 1-20. The added hydrogen serves to remove carbon deposits formed on the surface of the catalyst.
Apart from the continuous addition of a gas which prevents carbon deposits during the reaction, it is possible to regenerate the catalyst by passing hydrogen or air over it from time to time. The regeneration itself takes place at 300-900xc2x0 C., preferably 400-800xc2x0 C., using a free oxidizing agent, preferably using air, or in a reductive atmosphere, preferably using hydrogen. The regeneration can be carried out at subatmospheric, atmospheric or superatmospheric pressure. Preference is given to pressures in the range from 500 mbar to 100 bar.